The present invention relates to a kiln plant and a method for manufacturing cement. The kiln plant comprises a kiln for burning clinker, a cooler, a calciner followed by a separation cyclone and a burning compartment which is fed tangentially with tertiary air from the cooler. Completely or partially calcined material is routed from the bottom of the burning compartment to the calciner to which exhaust gases from the kiln are also vented.
It is a recognized fact that a number of advantages may be achieved by constructing a calcination zone equipped with an extra burning compartment which is exclusively fed with tertiary air from the cooler.
A plant of this type is described in our EP-A-103423. From this patent is known a plant (SLC-S) for calcination of cement raw materials, in which due allowance has been made for the inherent difficulty in achieving a complete burn-out of the fuel which is utilized in the calciner. In this plant the raw material is fed, after being preheated, to a burning compartment (4) in which it is calcined in hot air from the cooler (2). The tertiary air from the cooler flows centrally up through the bottom of the burning compartment (4) which means that there is an upward flow centrally in the burning compartment and a downward flow along the sides of the burning compartment. The raw material is fed at the lower end of the burning compartment and is distributed subject to the action of the turbulent flow encountered across the entire length of the burning compartment.
From U.S. Pat. No. 4,014,641 is known a plant for the calcination of cement raw materials in which the quantity of nitrogen oxide in the exhaust gases of the kiln is reduced by generating a zone in the kiln discharge duct to which reducing gas is fed. Hot air from the cooler and hot gas from the kiln are via a duct (5) and a duct (13), respectively, routed to a cyclone preheater (14,15,16,17) in which raw material is preheated in counter-current to the hot gas from the cooler and from the kiln. In the area of the kiln discharge duct which is located below the feed duct (5) from the cooler, reducing conditions are generated by introducing reducing gases via a duct (12). The reducing gases are formed in the calciner (8) since the air volume which is fed to the calciner is sufficient to cause gasification of the fuel in the calciner but insufficient to cause complete burn-out of the fuel in the calciner (col. 4, lin. 1-5). A disadvantage of this plant is that fuels which are difficult to ignite and slow-burning such as petcoke, anthracite and other grades of coal with a low gas content cannot be utilized in the calciner since they would produce a substantial unburned coke residue which would be precipitated and led to the rotary kiln, subsequently giving rise to problems in terms of sulphur expulsion and formation of cakings.
From U.S. Pat. No. 5,364,265 is known yet another calcining system in which the NO.sub.x emission is limited by formation of reducing gases, viz. Co and H.sub.2, in a burning compartment (20). The coke formed in the burning compartment during this process exhibits quite distinctive reactive properties. However, optimization of this method is relatively difficult to achieve in respect of ensuring minimum NO.sub.x emission given that very few parameters can be controlled during operation. The firing process in the burning compartment depends entirely on the desired degree of calcination of the raw meal. At the same time, the complexity of the plant structure is relatively high.
From our international patent application No. PCT/DK 97/00029 is known a method for reducing the NOx rate of emission from a plant. The object of this invention is to provide a method for operating a kiln plant with a reduced NO.sub.x emission rate, while, simultaneously, providing the option of using fuel with a low reactivity, such as petcoke, anthracite and other grades of coal with a low gas content in zones with a relatively low temperature. This object is obtained by regulating the fuel input rates in three different zones, with exhaust gases being fed to one zone from the two other zones in a way ensuring minimization of the NO-content in the exhaust gases from the zone to which exhaust gases are vented from the two other zones.
The object of the present invention is to provide a kiln plant and a method for manufacturing cement. The kiln plant is of a relatively simplified construction and it may advantageously burn fuel with a low reactivity, such as petcoke, anthracite and other grades of coal with a low gas content in the calcining zone where the temperature is generally at a low level.